• Title/Summary/Keyword: Chemical density

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Influence of Thermal Aging in Change of Crosslink Density and Deformation of Natural Rubber Vulcanizates

  • Choe, Seong Sin
    • Bulletin of the Korean Chemical Society
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    • v.21 no.6
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    • pp.628-634
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    • 2000
  • Crosslink is the most important chemistry in a rubber vulcanizate. Degree and type of crosslinks of the vulcanizate determine its physical properties. Change of crosslink density and deformation of a rubber vulcanizate by thermal aging were studied using natural rubber (NR) vulcanizates with various cure systems (conventional, semi-EV, and EV) and different cure times (under-, optimum-, and overture). All the NR vulcanizates were deformed by the thermal aging at 60-100 $^{\circ}C.$ The higher the aging temperature is, the more degree of the deformation is. The undercured NR vulcanizates after the thermal aging were deformed more than the optimumand overcured ones. The NR vulcanizates with the EV cure system were less deformed than those with the conventional and semi-EV cure systems. The deformation of the NR vulcanizates was found to be due to change of the crosslink density of the vulcanizates. The crosslink densities of all the vulcanizates after the extraction of organic materials were also changed by the thermal ging. The sources to change the crosslink densities of the vulcanizates by the thermal aging were found to be dissociation of the existing sulfur crosslink and the formation of new crosslinks by free sulfur, reaction products of curing agents, and pendent sulfide groups.

Relation Between the Repulsive Interaction and the Overlap of the Electron Densities$^\dag$

  • Heo, Hoon;Shin, Kook-Joe;Kim, Yung-Sik
    • Bulletin of the Korean Chemical Society
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    • v.10 no.3
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    • pp.238-242
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    • 1989
  • The relations between the repulsive interactions and the electron density overlaps are investigated for various closed shell-closed shell pairs, including the systems containing alkali and halide ions. It is found that the repulsive interaction($V_{rep}$) depends on the overlap of the electron density($S_{\rho}$) according to a simple exponential relation, $V_{rep}$ = $As_{\rho}\;^{\alpha}$. Furthermore, for most of the closed shell systems the $\alpha$ values are near unity and the A values do not vary much. The same tests are also performed for the open shell-closed shell, and the open shell-open shell pairs. Although the results for these systems also show exponential dependences of the repulsive interactions on the density overlaps, the details of the dependence differ greatly from those for the closed shell systems and also vary widely from one individual system to another.

Theoretical Investigation of the Hydrogen-bonded Halide-acetylene Anion Complexes

  • Byeong-Seo Cheong
    • Journal of the Korean Chemical Society
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    • v.68 no.2
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    • pp.65-73
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    • 2024
  • The halide-acetylene anions, X--HCCH (X = F, Cl, and Br) have been studied by using several different ab initio and DFT methods to determine structures, hydrogen-bond energies, vibrational frequencies of the anion complexes. Although the halide-acetylene complexes all have linear equilibrium structures, it is found that the fluoride complex is characterized with distinctively different structure and interactions compared to those of the chloride and bromide complexes. The performance of various density functionals on describing ionic hydrogen-bonded complexes is assessed by examining statistical deviations with respect to high level ab initio CCSD(T) results as reference. The density functionals employed in the present work show considerably varying degrees of performance depending on the properties computed. The performances of each density functional on geometrical parameters related with the hydrogen bond, hydrogen-bond energies, and scaled harmonic frequencies of the anion complexes are examined and discussed based on the statistical deviations.

Sintering and Grain Growth of Rare Earth-Doped Ceria Particles

  • Sameshima, Soichiro;Higashi, Kenji;Hirata, Yoshihiro
    • Proceedings of the Korea Association of Crystal Growth Conference
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    • 2000.06a
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    • pp.65-86
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    • 2000
  • Rare earth-doped ceria powders with a composition of Ce0.8R0.2O1.9(R=Yb, Y, Gd, Sm, Nd and La) were prepared by heating the oxalate coprecipitate. The green compacts began to shrink at 600$^{\circ}$-700$^{\circ}C$. The relative density after the sintering at 1200$^{\circ}$ and 1400$^{\circ}C$ became higher for the higher green density. The samples were densified above 98% relative density by the sintering ant 1600$^{\circ}C$ for 4 h and the grain sizes (4.7-7.6$\mu\textrm{m}$) showed a tendency to become larger with increasing ionic radius of doped-rare earth element. In the intial stag of sintering at 700$^{\circ}$-800$^{\circ}C$, the dominant mass transport process changed from lattice diffusion to grain boundary diffusion to grain boundary diffusion with heating time. The porosity during the intermediated and final stage of the sintering at 1200$^{\circ}$ and 1400$^{\circ}C$ decreased by the mass transport through lattice diffusion with grain growth.

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Stability Analysis of Wakes with Chemical Reaction (연소 반응을 가지는 후류 유동의 불안정성)

  • 신동신;홍성제
    • Journal of the Korean Society of Propulsion Engineers
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    • v.2 no.2
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    • pp.30-37
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    • 1998
  • This paper investigates the linear stability of wakes with special emphasis on the effect of chemical reaction. Velocity and density profiles for laminar flows are obtained from analytic profiles as well as from simulation. Wakes have two generalized inflection points and two unstable modes-sinuous and varicose modes. For analytical laminar profiles, sinuous modes are more unstable than varicose modes irrespective of density variation, which shows wakes will be destabilized by sinuous modes. Large velocity difference and density difference lead to more unstable wakes due to large momentum difference. For simulated laminar profiles, chemical reaction with stoichiometric chemistry increases temperature and stabilizes the flow due to increase in compressible reacting wades, flow becomes stable as velocity increases due to viscous dissipation.

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Quantum Chemical Designing of Efficient Sensitizers for Dye Sensitized Solar Cells

  • Abdullah, Muhammad Imran;Janjua, Muhammad Ramzan Saeed Ashraf;Mahmood, Asif;Ali, Sajid;Ali, Muhammad
    • Bulletin of the Korean Chemical Society
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    • v.34 no.7
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    • pp.2093-2098
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    • 2013
  • Density functional theory (DFT) was used to determine the ground state geometries of indigo and new design dyes (IM-Dye-1 IM-Dye-2 and IM-Dye-3). The time dependant density functional theory (TDDFT) was used to calculate the excitation energies. All the calculations were performed in both gas and solvent phase. The LUMO energies of all the dyes were above the conduction band of $TiO_2$, while the HOMOs were below the redox couple (except IM-Dye-3). The HOMO-LUMO energy gaps of new design dyes were smaller as compared to indigo. All new design dyes were strongly red shifted as compared to indigo. The improved light harvesting efficiency (LHE) and free energy change of electron injection ${\Delta}G^{inject}$ of new designed sensitizers revealed that these materials would be excellent sensitizers. The broken coplanarity between the benzene near anchoring group having LUMO and the last benzene attached to TPA unit in all new design dyes consequently would hamper the recombination reaction. This theoretical designing will the pave way for experimentalists to synthesize the efficient sensitizers for solar cells.

Fabrication of La2O3-TiO2-SiO2 System Glass Derived from a Sol-Gel Process

  • Iwasaki, Mitsunobu;Masaki, Hitoshi;Ito, Seishiro;Park, Won-Kyu
    • Journal of the Korean Ceramic Society
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    • v.44 no.3 s.298
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    • pp.137-141
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    • 2007
  • $La_{2}O_{3}-TiO_{2}-SiO_{2}$ glass, a type that could not obtained so far by the conventional melting method, was prepared successfully using a sol gel process. Glass derived with the sol-gel process has compositions of $5La_{2}O_{3}-5TiO_{2}-90SiO_{2},\;5La_{2}O_{3}-10TiO_{2}-85SiO_{2}$, and $5La_{2}O_{3}-20TiO_{2}75SiO_{2}$. The UV-visible absorption edge of all glass compositions was below 400 nm. The measured density is in the range of 2.55-2.89, and was nearly identical to the calculated density and the refractive index of the glasses derived from the sol-gel ranges from 1.545 to 1.645. The molar additive coefficient of $TiO_{2}$ measured in this ternary system is lower than the calculated value, while the value of $La_{2}O_{3}$ is higher.

Facile Synthesis of Vertically Aligned CdTe-Si Nanostructures with High Density (수직배양된 고집적 CdTe-Si 나노구조체의 제조방법)

  • Im, Jinho;Hwang, Sung-hwan;Jung, Hyunsung
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.30 no.3
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    • pp.185-191
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    • 2017
  • Cadmium compounds with one dimension (1D) nanostructures have attracted attention for their excellent electrical and optical properties. In this study, vertically aligned CdTe-Si nanostructures with high density were synthesized by several simple chemical reactions. First, l D Te nanostructures were synthesized by silver assisted chemical Si wafer etching followed by a galvanic displacement reaction of the etched Si nanowires. Nanowire length was controlled from 1 to $25{\mu}m$ by adjusting etching time. The Si nanowire galvanic displacement reaction in $HTeO_2{^+}$ electrolyte created hybrid 1D Te-branched Si nanostructures. The sequential topochemical reaction resulted in $Ag_2Te-Si$ nanostructures, and the cation exchange reaction with the hybrid 1D Te-branched Si nanostructures resulted in CdTe-Si nanostructures. Wet chemical processes including metal assisted etching, galvanic displacement, topochemical and cation exchange reactions are proposed as simple routes to fabricate large scale, vertically aligned CdTe-Si hybrid nanostructures with high density.

Interactive CO2 Adsorption on the BaO (100) Surface: A Density Functional Theory (DFT) Study

  • Kwon, Soon-Chul;Hwang, Jung-Bae;Lee, Han-Lim;Lee, Wang-Ro
    • Bulletin of the Korean Chemical Society
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    • v.31 no.8
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    • pp.2219-2222
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    • 2010
  • A density functional theory (DFT) study of $CO_2$ adsorption on barium oxide (BaO) adsorbents is conducted to understand the chemical activity of the oxygen site on the BaO (100) surface. This study evaluated the adsorption energies and geometries of a single $CO_2$ molecule and a pair of $CO_2$ molecules on the BaO (100) surface. A quantum calculation was performed to obtain information on the molecular structures and molecular reaction mechanisms; the results of the calculation indicated that $CO_2$ was adsorbed on BaO to form a stable surface carbonate with strong chemisorption. To study the interactive $CO_2$ adsorption on the BaO (100) surface, a pair of $CO_2$ molecules was bound to neighboring and distant oxygen sites. The interactive $CO_2$ adsorption on the BaO surface was found to slightly weaken the adsorption energy, owing to the interaction between $CO_2$ molecules.

Structure-property relations for polymer melts: comparison of linear low-density polyethylene and isotactic polypropylene

  • Drozdov, A.D.;Al-Mulla, A.;Gupta, R.K.
    • Advances in materials Research
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    • v.1 no.4
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    • pp.245-268
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    • 2012
  • Results of isothermal torsional oscillation tests are reported on melts of linear low density polyethylene and isotactic polypropylene. Prior to rheological tests, specimens were annealed at various temperatures ranging from $T_a$ = 180 to $310^{\circ}C$ for various amounts of time (from 30 to 120 min). Thermal treatment induced degradation of the melts and caused pronounced decreases in their molecular weights. With reference to the concept of transient networks, constitutive equations are developed for the viscoelastic response of polymer melts. A melt is treated as an equivalent network of strands bridged by junctions (entanglements and physical cross-links). The time-dependent response of the network is modelled as separation of active strands from and merging of dangling strands with temporary nodes. The stress-strain relations involve three adjustable parameters (the instantaneous shear modulus, the average activation energy for detachment of active strands, and the standard deviation of activation energies) that are determined by matching the dependencies of storage and loss moduli on frequency of oscillations. Good agreement is demonstrated between the experimental data and the results of numerical simulation. The study focuses on the effect of molecular weight of polymer melts on the material constants in the constitutive equations.